Sometimes Firefox can bite you in the hind quarter. If you are testing out your new apache2 / php setup and finding Firefox downloads the php instead of running it through apache, and you have checked all the configs (and possibly changed some!):
Check this as well:
The Firefox offline storage cache can be persistent between runs and need cleaning out before you hit the script again (i.e. it will continue to "download" that which is in your offline storage instead of fetching new content.) Kinda makes me want to test more with Lynx sometimes.